The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950.degree. C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Heroult, has not evolved as many other electrochemical processes.
The anodes are still made of carbonaceous material and must be replaced every few weeks. The operating temperature is still not less than 950.degree. C. in order to have a sufficiently high solubility and rate of dissolution of alumina and high electrical conductivity of the bath.
The carbon anodes have a very short life because during electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form polluting CO.sub.2 and small amounts of CO and fluorine-containing dangerous gases. The actual consumption of the anode is as much as 450 Kg/Ton of aluminium produced which is more than 1/3 higher than the theoretical amount of 333 Kg/Ton.
The frequent substitution of the anodes in the cells is still a clumsy and unpleasant operation. This cannot be avoided or greatly improved due to the size and weight of the anode and the high temperature of operation.
Several improvements were made in order to increase the lifetime of the anodes of aluminium electrowinning cells, usually by improving their resistance to chemical attacks by the cell environment and air to those parts of the anodes which remain outside the bath. However, most attempts to increase the chemical resistance of anodes were coupled with a degradation of their electrical conductivity.
U.S. Pat. No. 4,614,569 (Duruz et al.) describes anodes for aluminium electrowinning coated with a protective coating of cerium oxyfluoride, formed in-situ in the cell or pre-applied, this coating being maintained by the addition of cerium to the molten cryolite electrolyte. This made it possible to have a protection of the surface only from the electrolyte attack and to a certain extent from the gaseous oxygen but not from the nascent monoatomic oxygen.
EP Patent application 0 306 100 (Nyguen/Lazouni/Doan) describes anodes composed of a chromium, nickel, cobalt and/or iron based substrate covered with an oxygen barrier layer and a ceramic coating of nickel, copper and/or manganese oxide which may be further covered with an in-situ formed protective cerium oxyfluoride layer.
Likewise, U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan) disclose aluminium production anodes with an oxidised copper-nickel surface on an alloy substrate with a protective barrier layer. However, full protection of the alloy substrate was difficult to achieve.
A significant improvement described in U.S. Pat. No. 5,510,008, and in International Application WO96/12833 (Sekhar/Liu/Duruz) involved micropyretically producing a body from nickel, aluminium, iron and copper and oxidising the surface before use or in-situ. By said micropyretic methods materials have been obtained whose surfaces, when oxidised, are active for the anodic reaction and whose metallic interior has low electrical resistivity to carry a current from high electrical resistant surface to the busbars. However it would be useful, if it were possible, to simplify the manufacturing process of these materials and increase their life to make their use economic.
U.S. Pat. No. 5,725,744 (de Nora/Duruz) describes aluminium electrowinning cells provided with an upward circulation of electrolyte by gas lift between the electrodes which can be porous or reticulated skeletal anode structures of coated metal having a high active surface area and allowing for internal electrolyte circulation and gas release.
Metal or metal-based anodes are highly desirable in aluminium electrowinning cells instead of carbon-based anodes. As described hereabove, many attempts were made to use metallic anodes for aluminium production, however they were never adopted by the aluminium industry because of their poor performance.